1. Field of the Invention
This invention relates to impact printing and in particular to impact printers in which dots are recorded on a print medium to form images, lines, symbols or the like.
2. Discussion of the Prior Art
In an all-points addressable dot printer individual dots are recorded selectively at all addressable point positions in a continuous line of dots extending across a record medium. In order to produce recorded images of good print quality, the recorded dots must be precisely located and uniformly spaced at all addressable points of the line and it is desirable to be able to record successions of spaced dots as closely together as possible.
U.S. Pat. No. 2,205,450 issued to R. J. Wise, uses a single helical anvil on the rotating drum in combination with a single marking blade or print bar which extends entirely across the line of print. While capable of recording closely spaced dots with uniform spacing, this type of printer is speed limited. This is particularly true where the print lines are guite long.
U.S. Pat. No. 3,138,429, issued to A. G. Cooley, provides a mechanism for printing at somewhat higher speeds by providing an anvil with a plurality of convolutions. A single marking blade is transversely flexible and can record dots simultaneously at several spaced intervals across the line. The transversely flexed marking blade and its operating mechanisms are complex structures. Additionally, the convolutions of the anvil must be relatively widely separated to avoid shadow printing from adjacent sections of a single flexible bar when making contact with the anvil.
Much higher recording speeds have been achieved by using plural separate and individually operable marking blades aligned in a single row. A helical anvil on a rotating cylinder has a large number of convolutions. Each convolution is spanned by a single hammer. Examples of such printers are shown in U.S. Pat. Nos. 3,409,904, issued to K. Maiershoffer; 3,810,195, issued to H. P. Kilroy, et al; 3,813,492, and 3,830,975, both issued to J. T. Potter; 3,843,955, issued to C. B. Pear, Jr.
In the multiple blade and helix printers of the type disclosed in the above-mentioned references, a separation or gap exists between the blades to permit interference-free individual operation. The blade separation presents no problem for character printing since such printing naturally requires some separation between characters for legibility. However, in all-points addressable printing, the blade separation that must exist between the individual blades is a limiting factor on the density of the dots. Non-uniformity of the blade separation contributes to degraded print quality. Minimizing the dimension of blade separation and maintaining its uniformity between all print hammer blades requires costly structures and great care in assembly.
All-points addressable printers having also been provided using other structures. In U.S. Pat. No. 3,941,051, issued to Barrus, et al, uniformly spaced dot printing hammers on a common carrier are reciprocated along the print line the full length of a print segment. The amplitude of motion of the hammers is relatively large, thereby requiring large dynamic forces.